1. Field of Invention
The present invention relates to an electrostatic discharge protection circuit (ESD protection circuit). More particularly, the present invention relates to a diode of preferred electrostatic conductivity and an ESD protection circuit composed thereof.
2. Description of Related Art
Multimedia technology in present society is quite prosperous mainly due to the progress the semiconductor and display technology. As for the display, the liquid crystal display (LCD), with advantages such as high picture quality, good space utilization, low power consumption, and no radiation, has gradually become the mainstream of the market. Generally, during the fabricating process of an LCD, operators, machines, and inspection instruments may all carry static electricity. Therefore, when the above charged bodies (operators, machines, or inspection instruments) make contact with an LCD panel, the devices and circuits in the LCD panel may be damaged by electrostatic discharge.
To avoid the devices and circuits in the LCD panel from being damaged by electrostatic discharge, an ESD protection device is usually formed on the thin film transistor (TFT) array substrate of the LCD panel, wherein the ESD protection device is generally formed on the substrate together with the TFT and a pixel electrode. In general, before the ESD protection device is formed, a TFT with three terminals is formed on the substrate, and then the gate and drain in the TFT are electrically connected to each other. At this time, the TFT with its gate and drain electrically connected is equivalent to a diode with two terminals, and the diode can be used as an ESD protection device.
Referring to FIG. 1, it shows the schematic sectional view of a conventional diode used as an ESD protection device. The structure of a conventional diode 100 is similar to a bottom gate TFT. It can be seen from FIG. 1 that, the diode 100 includes a gate 121, a gate insulating layer 122, a channel layer 123, an ohmic contact layer 124, a source 125, a drain 126, a protection layer 127, and a transparent electrode 128.
The gate 121 is disposed on a substrate 110, and the gate insulating layer 122 covers the gate 121 on the substrate 110. Moreover, the channel layer 123 is disposed on the gate insulating layer 122 above the gate 121. It can be seen from FIG. 1 that, the source 125 and the drain 126 are respectively disposed at both sides of the channel layer 123 and cover a portion of the insulating layer 122. The ohmic contact layer 124 is generally formed between the source 125 (or the drain 126) and the channel layer 123 to reduce the contact impedance between the source 125 (or the drain 126) and the channel layer 123.
Besides, the protection layer 127 covers the source 125, drain 126, channel layer 123, and gate insulating layer 122. It can be seen from FIG. 1 that, before the source 125, drain 126, and channel layer 123 are formed, a contact window 10 is formed in the protection layer 127 and the gate insulating layer 122 to expose a portion of the gate 121. Besides, another contact window 20 is formed in the protection layer 127 to expose a portion of the drain 126. The contact windows 10 and 20 are filled with a portion of above-mentioned transparent electrode 128, whereby it is electrically connected to the gate 121 via the contact window 10, and is electrically connected to the drain 126 via the contact window 20.
When the electrostatic discharge phenomenon occurs, the foregoing ESD protection device 100 can consume the electrostatic energy and lead the static electricity out, so as to prevent the devices in the LCD panel from being damaged by electrostatic discharge. As the electrostatic discharge protecting ability of the ESD protection device 100 is limited, the producers fabricate enough ESD protection devices 100 on the substrate 110 to prevent the devices and circuits in the LCD panel from being damaged by electrostatic discharge. However, it may relatively reduce the area of the substrate 110 available for layout. Further, if the gate 121 in the ESD protection device 100 is damaged or disconnected, the whole ESD protection device 100 will stop operating.